1. Field of the Invention
This invention relates to helical antennas and more particularly to an improved method and construction for such an antenna to permit greater precision of construction and more reliable duplication of antenna characteristics.
2. Description of the Related Art
Both tunable and fixed helical antennas of various types and configurations are known. Usually these antennas have been tuned by short circuiting part of the turns of the helix or by disconnecting a portion of the helix from the antenna circuit. This has been accomplished by electro-mechanical switching means.
Such antenna structures are generally capable of switching at only relatively slow rates, have restricted bandwidth over which the antenna can be tuned, and are generally expensive to construct. Precise tuning is difficult and precision reproducibility is impossible.
The above-referenced application discloses an electrically-tunable helical antenna making use of electronically controlled diode networks that tune the antenna quickly and precisely which are connected across sections of the helical windings in such a manner as to prevent an substantial interference with the operation of the antenna by the control networks. Balanced pairs of diodes of opposing polarity are connected across sections of the helix which when forward biased short-circuit selected sections of the helix to change the electrical length of the antenna. The bias feed lines for operation of the diode are carried within the helix turns, as by using a coaxial cable, and thus are prevented from interfering with the rf currents.